1. Field of the Invention
This invention pertains to a cementitious composition for use as backfill and annulus fill for underground pipelines constructed by boring, particularly or use in soil conditions wherein substantial ground water is encountered.
There are many types of underground structures in which cementitious fill materials are used, either as the primary solid or as a binder for aggregates. The design and selection of suitable cementitious material is perhaps more difficult than for surface structures because of the variable conditions that can occur, and for which there is often only limited information. Tunnels, pipelines, oil wells, piles, caissons, control rooms, mines, piers, dams, and earth slide areas are examples. Construction must cope with ground water, unstable and low bearing capacity soils, subsoil voids and caverns, lithostatic pressures, corrosive soil chemicals, difficult placement conditions, often remote from the surface, high temperatures and many other conditions not encountered in monolithic structures built on the surface.
2. Description of the Prior Art
The terms "cementitious material" and "cement," as used herein, means compounds that in contact with water react therewith and undergo a crystalline transformation. Examples would be the various types of Portland cement, certain autoclaved gypsum stuccos, high alumina cements, pozzolana cements, magnesia cements and the like. In this hydration process, the newly formed crystals interlock to become a rigid, continuous mass. Such materials can be used by themselves to make structures such as floors, walls, beams, pipe and a vast number of well-known items, but more often, mainly for cost considerations, they are used as binders of inexpensive filler materials such as sand and gravel. When cementitious materials are used with sand only, the mixture is commonly called "grout" or "mortar." If sand and gravel both are used, the mixture is called "concrete." If the cementitious material alone is used, it is termed "neat."
Since the conditions in which the cementitious compositions must be placed in underground construction vary widely and are often unpredictable, the design of mixes that can flow freely through long lengths of conduit and through forms or earth spaces that may cause the mix to dilute or dewater, or both, is critical.
The backfilling of tunnels and the filling of the annular space between the liners used in bore-type tunnels and the pipe or conduit set therein often involves a variety of requirements. A high degree of fluidity or flowability is desired, so as to minimize the number of downholes or injection holes needed to insure complete filling of the space. Limiting the attainment of high fluidity is the prerequisite to maintain the lowest possible water content for strength considerations and for reasons of preventing stratification, since any water layer at the top of the cast resulting from settlement of the dense aggregates and cement will become a void at the point where the soil overburden requires greatest support.
A very critical consideration is the presence of ground water, either ponded in the pour cavity or flowing therein. If the concrete or cementitious material is made fluid by use of a high water/cement ratio, in order to facilitate placement, it thereby also becomes more susceptible to dilution as it flows through ponded water, and flowing ground water will leach out the cementitious material as well as other fines such as sand or fly ash. The true strength of the final set cast thus becomes variable, as is the extent to which the cavity is actually filled after the excess water rises to the top when the concrete comes to rest. If, as an alternative, a fairly stiff, 2-3 inch slump concrete, cohesive enough to resist dilution by water, is used, this limits the distance the concrete will flow, hence requires a greater number of expensive downholes or pump hose line input points.
The placement of tunnel fills differ from the well-known technique of tremie placement. Generally tremie concreting involves the filling of a vertical cavity, and the hose or pipe extends to and discharges at the bottom, so that any water standing in the vertical cavity is lifted by the denser concrete and only a few inches of the top surface of the concrete placement is degraded by the water. Tunnel backfilling usually involves downflow from the surface also, but then the fill material must flow horizontally through irregular, often narrow, cross sections, one face of which may be soil of unpredictable texture. If groundwater is present, it will be ponded or will be flowing in the lower portions of the cavity. Even if the incoming concrete is displacing ponded still water, it is obvious that the leading edge of the concrete is subjected to much more washing and leaching action than in the case of tremie placement, and the extent of degradation will be much more severe. Even more serious, however, are cases in which there is a continuous flow of groundwater into the cavity in spite of the use of external dewatering pumps. This adds the erosive and diluting action of running water to that of the water lying on the cavity bottom. The placement of the cementitious composition is often impaired by conditions the reverse of the above. The horizontal flow of the composition may be inhibited or even prevented by its passage through dry soil or other surfaces that dewater the composition and reduce it to a semisolid, nonflowing state. The general approach to preventing this is to add a water thickening agent. Thickeners are gelling compounds that physically entrap water within their long-chain, convolute molecular clusters, or to some degree absorb water, sometimes with weak hydrogen-hydroxyl bonding. The long-chain molecules also act like dispersed fibers to increase the fluid viscosity of the solution. Gum arabic and gelatin are examples of water thickeners. For oil well cementing, commercial type agents are used. Thus R. A. Salathiel, U.S. Pat. No. 2,582,459, employed bentonitic and montmorillonite clay in 1952, as did John V. Drummond, U.S. Pat. No. 2,876,123, 1959. The development of synthetic thickeners was exploited by Robert C. Martin, U.S. Pat. No. 3,234,154, 1966, using sulfonated polyvinyl styrene and polyvinyl toluene, and Charles F. Weisend, U.S. Pat. No. 3,132,693, 1964, and U.S. Pat. No. 3,359,225, 1967, using hydroxyethyl cellulose and polyvinylpyrrolidone. There are several other such agents used in the art.
All of the thickeners heretofore used in oil well cementing dissolve and fully disperse relatively rapidly in water. Their thickening action increases the viscosity of the cementitious composition, an effect that can be countered by the addition of a dispersing agent which would, in the absence of the thickener, increase the fluidity of the mix. To some degree, the gelatinizing of the water imparts lubricity, since the presence of the gel, and its coating of the angular solid particles, reduces the friction between the slurry and the conduit or medium into which the slurry is flowing. The major objective, however, is to "thicken" the water and reduce its ability to be absorbed or dry surfaces with which it comes in contact during placement.
Foamed or cellular concrete can be used as tunnel backfill under certain conditions. The limitation is that there be no ground water seepage in the cavity or any ponded or impounded free water, since water will disintegrate the foam of the mix and result in collapse separation into a top layer of foam, a middle layer of water and cement last at the bottom. Cellular or light-weight aggregate concrete is useful and economical in the 25 to 50 pounds per cubic foot density, giving strengths of 150 to 400 pounds per square inch, because such material pumps and flows readily over long distances. Obviously, however, such a composition will float on a 62.5 pounds per cubic foot liquid, such as water, hence cannot be used except in "dry" tunnels. At higher densities that could displace water, e.g. 70 pounds per cubic foot or more, the cost is almost double that of the conventional mortars and concretes currently used.
Plasticizers, or water-reducing agents, have been used in cementitious compositions for many years. They are polymeric polyelectrolyte compounds that bond to the surfaces of finely comminuted solids, including many that are not cementitious, and create an enhanced negative surface charge thereon. Since all the particles thus become like charged, they tend to repel each other. This results in deagglomeration of clusters or particles, and the separated, dispersed particles are much more mobile in the aqueous medium than are the large, angular clumps. The tiny particles are themselves angular, and normally tend to interlock somewhat so as to reduce the fluidity of the slurry. When strongly surface charged, they repel each other so as to provide space for free rotation and movement in the water of slurry, resulting in greater fluidity. The user may confine his benefits from such additives to the higher slump, or workability, thus obtained without degrading the composition by adding excess water, or he may elect to reduce the amount of mixing water used, thereby improving the quality of his cementitious composition while maintaining normal workability.
The most widely used plasticizing agents are the lignosulfonates, by-products of the paper industry. The principle objection to them is that they retard the rate of hydration, or hardening, of cementitious materials, hence can be used only in limited dosage. More recently, two synthetic polymers have found increasing popularity, mainly because they induce a higher level of surface charge and because they do not adversely affect the hydration reaction. The compounds are sodium naphthalene sulfonate (monomer) condensed (polymerized) with formaldehyde (U.S. Pat. No. 2,141,569 to George R. Tucker, 1938) and sodium melamine sulfonate condensed with formaldehyde, developed in Germany. It is at least theoretically possible to develop other equally effective plasticizers beginning with various monomers. Such compounds are designated as "superplasticizers," or "high range water reducing agents" in current terminology. A major advantage to their use, over lignosulfonates, is that they cause only a slight retardation of cement hydration.
As noted, the superplasticizers can be used to maintain fluidity in a composition even when 30% to 50% less water is used. Their merit in tunnel backfilling is in providing high fluidity as normal or low water contents. As elsewhere discussed, this significantly reduces segregation and "bleeding," the formation of a water layer on the top of the cast that eventually becomes a weakening void. A further advantage is that as the water content of a cementitious composition is reduced the rate of hydration or hardening is proportionately increased, as is well understood in the art. The compositions generally used in tunnel backfilling are preferrably made as low as possible in the expensive cement fraction, hence normally develop strength slowly when made with the usual high water contents of present art. If the water content is reduced and the fluidity maintained by use of superplasticizer, the composition will gain strength much more rapidly and attain much higher strength levels. This can be exploited by reducing the expensive cement fraction of the composition to a level that has the same performance characteristics as the high water mixture.
In the present invention, the enhanced fluidity is further exploited to permit the use of high dosages of the pituitous water thickening agent, to levels that would otherwise cause the composition to become too stiff for rapid flowing in the tunnel cavities.